Certain contractile properties of the human mandibular muscles are unknown. These include twitch contraction time for geniohyoid, mylohyoid, and both heads of lateral pterygoid, and the characteristic length-tension relations for those muscles, as well as for masseter, temporalis, medial pterygoid, and anterior digastric. The techniques of single-motor-unit electromyography, force transduction, and spike-triggered averaging will be applied to human subjects to determine in vivo (1) the characterisitc relations between twitch contraction time, twitch tension, and recruitment threshold for single motor units in each of these muscles; (2) the effects of gape, and the attendant changes in muscle length, on motor unit twitch contraction times and twitch tensions; and (3) the extent of homogeneity of contractile properties of motor units having similar "sizes" but lying in different regions of each muscle. It is well known that contractile properties of whole muscles are reasonably well-predicted from averages of properties of their motor units. Thus, the data to be obtained will provide a first characterization of properties of the human mandibular muscles which constrain their capabilities for generating and modulating force. Presumably, properties such as these will also constrain the individual functions and temporal coordination of these muscles during execution and control of complex movements of the mandible which occur in speech and mastication. Thus, data describing these properties are of fundamental importance for a general biomechanical model of this portion of the human musculoskeletal system. Such a model would be particularly useful in investigation of both normal and disordered speech production, and may prove of equal value in predicting certain functional consequences of dental and craniofacial surgical procedures which affect the mandibular muscles.